Refractory reservoir for aluminum



June 17, 1958 H. T;`BE|.LAMY

RERRAcToRY RESERVOIR FoR ALUMINUM Filed Aug. 9, 1954 United StatesPatentO .l 2,839,292 "REFRACTORY RESERVOIR FOR ALUMINUM i Harry T.Bellamy, Evanston, Ill.

f Application August 9, 1954, serial No. 443,609rv a claims. (ci.zas-s4) This invention relates to refractory articles or parts that are'resistant to corrosion by molten aluminum and to a method for makingsuch articles. l t

At its melting point of 12,16 F. orabove, aluminum is highly corrosiveand rapidly attacks common structural metals. Tubing of iron, ferrousalloys, alloys of chromium, nickel and copper, for example, when dippedinto molten aluminum will pit and develop holes completely through thewall thereof Within a relatively short time. Attempts to coat or platethese metals and alloys to protect them from this corrosivek action havebeen disappointing.` Common plating metals, such as chromium landnickel, are vigorously attacked by molten aluminum. Ceramiccompositions, although resistant to molten aluminum, cannot successfullybe applied in a continuous film, primarily due to difference in thecoecient of thermal expansion between the ceramic and the metal base.Invariably some surface defect develops, such as pinholes or cracks inthe coating, to blemish the continuity of the surface, and attack by themolten aluminum results at these points. jConsequently, parts which mustbe capable of resisting attack by molten aluminum are conventionallymade from cast or otherwise shaped solid ceramic compositions. Pyrometerprotecting tubes, for example, are formed by extruding a refractoryceramic which contains a high proportion of siliconcarbide with asuitable bonding material.

Other parts adapted to resist molten made from graphite. parts stand upfairly well under the corrosive action of aluminum, but aredisadvantageous in that they are brittle and break very easily duringinstallation or other handling. Graphite has the additional disadvantageof quickly oxidizing at elevated temperatures. These nonmetallic. partsare also rather difficult and expensive to fabricate as compared withferrous metals, for'example. The aluminum casting industry has been inneed of a suitable material that will withstand molten aluminum andwhich at the same time does not have the shortcomings of the brittlenon-metallic parts now commonly used.

I Vhave discovered that molybdenum and tungsten olfer surprisingresistance to corrosion by molten aluminum and that these metals serveadmirably as a coating for materials that are normally subject to attackby molten aluminum.y These metals are highly refractory, molybdenumhaving a melting point of 4750 F., and tungsten having a melting pointof 6l70 F. That molybdenum and tungsten should be capable of resistingattack by molten aluminum was especially surprising in view of the factthat chromium, which is also in group Vl of the periodic table, andwould predictably react chemically in the same manner as molybdenum andtungsten, is vigorously disintegrated by molten aluminum.

Of the two operable metals, molybdenum is the most vreadily availableand the easier to fabricate, and it is, therefore, preferred overtungsten. Molybdenum has been known and commercially available for manyyears,

aluminum are IKC@ 2 but so far asl am aware, no one previouslyappreciated its remarkable resistance to the corrosive action of moltenaluminum.

My invention consists in rendering parts or articles that are normallyattacked by molten aluminum resistant to' such attack by coatingk tht.Lsurface thereof with molybdenum or tungsten. Molybdenum may beconveniently applied lby Well known processes including metallizing(spraying), plating or cladding. I have found that an especiallyconvenient means of applying a uniform continuous molybdenum coating isby spraying. When the part being sprayed is made from a ferrous metal orferrous alloy, the molybdenum bonds tenaciously to a clean, Warmsurface, Steel parts have been provided with a wear resistant coatingAof molybdenum by spraying for some time. When the molybdenum sprayedpart is to be subjected to thermal shock, Inconel, an alloy comprisingsubstantially 80% nickel, 12-l4% chromium and 6-8% iron, is preferred asthe base metal due to its similar coefficient of expansion.

Most refractory parts associated with an aluminum melting furnace or thelike are completely submerged in molten aluminum. However, some partsmay become exposed to air at temperatures ,approaching the temperatureprevailing within the furnace as, for example, when the level of themolten aluminum falls below its maximum height. Under such conditionsthe molybdenum coating will oxidize and lose its refractorinessandcorrosion re; sistance. Therefore, parts lsubject to oxidation arerst coated with molybdenum and then given an external coating of asuitable refractory material that will protect the metallic coating fromoxidation, such as a conventional ceramic coating composition, or amolybdenum alloy containing a minor proportion of chromium and/ ornickel. Ceramic compositions can be applied by dip v Vcoating orspraying an aqueous solution or suspensionv Both graphite and theyceramic thereof, or they may be applied by what is known as llamespraying. This process involves the use of a gun equipped with a hot gasflame through which the powdered ceramic material is blown, thusconverting the solid powder into fused flaming particles which depositon the part and solidify as a firmly adhering coating.' A

coating will provide protection for molybdenum against' oxygen in areaswhich become exposed to air upon re-l moval from molten aluminum. Inareas where the ceramic coating may fail, the extent of the oxidation isnot suiicient to permit any substantial attack by the aluminum becausethe oxidized areas are small and the exposure to aluminum is notcontinuous.

Various examples of my invention are illustrated in the accompanyingdrawings, in which:

Figure 1 is a vertical section through an electric melting furnacehaving a connecting tube constructed inaccordance with the invention;

FigureY 2 is a sectional View taken alongv the line` 2 2 of Figure 1;

chamber 14 interconnected by means of horizontallyspaced channels ortubes 18. A magnetic core 15. is arranged around the tubes and isenergized by alternating current windings (not shown) to induce amelting-current in the aluminum metal within the tubes. .Since all ofthe melting occurs within the tubes, the prevailing current density is.very high and, consequently, this portionl of the furnace deterioratesmuch more rapidly than Vthe melting and holding chambers. Theimprovement in the construction illustrated lies in the tube 18, whichis made from molybdenum metal.

One method of incorporating the molybdenum tube 18 into the furnace isto support it by a refractory block 20, which is cast around the tube.The supporting block 20 may be conveniently made from a mixture similarto the ceramic composition 156, which consists of Lumnite cement(commercial heat-,resistant hydraulic cement) and a suitable aggregate.If the enclosed tube is prepared separately in this'way, the bond to theceramic mass may be more carefully controlled.

Another method, however, consists in casting the ceramic composition 16around the tube 18 simultaneously with the formation of the melting andholding chambers.

The molybdenum tube will outlast theconventional ceramic tube manytimes, and, of course, is not subject to breakage. denum tube, it ispreferred that the molybdenum tube be coated inside and out prior toenclosure in cast composition 16, or in the block 20, with a suitableoxidation protective coating that will withstand temperatures in thevicinity of -1500 F., and which preferably will also resist moltenaluminum. A vapor-phase deposited silicon coating or other ceramiccoating, such as magnesium or calcium oxides, are suitable for thispurpose. In an installation of this kind the primary source of oxygen isdue to permeation of air through the ceramic composition 1,6. Theoxidation that might occur would not be severe, but it is preferable toprotect against it n the manner described.

Figure 3 shows a bubble tube which is commonly used in aluminumfoundries to introduce gases, such as nitrogen or chlorine, into moltenaluminum. The conventional tube is made from graphite and deterioratesrather rapidly due to oxidation and, of course, such tubes are highlybrittle and are frequently broken.

The bubble tube of Figure 3 has been made in accordance with theinvention and consists of a standard length of suitable diameterstandard iron pipe 32 threaded at one end and closed at the other. Theclosed lower end contains a suitable number of holes 36 for permittingthe escape of gas through the wall of the tube. The surface of the metalis cleaned thoroughly to remove any dirt or grease and a coating 33 ofmolybdenum, about .005 inch thick or more, is sprayed over the surface.To protect the molybdenum coating 33 from oxidation a ceramic coating 34is applied thereover by flame spraying or by applying a coating by othermeans. Thestandard ferrous metal base coated in this manner ispractically unbreakable since the layers are securely bonded together.It will resist attack by molten aluminum almost indelinitely. In use,the tube 35 is connected to the gas line 37 through an elbow 38, anddips into molten aluminum 31 in the refractory container 30.

To insure against oxidation of the molyb- Figures 4 and 5 illustrate anelectrical immersion heater 40 of conventional construction, which hasbeen treated according to the invention. The heater contains the usualelectrical resistor core 44 sheathed in an insulating material 45 and anexternal metal housing 46 which is normally subject to corrosion bymolten aluminum. This immersion unit may be protected from the action ofmolten aluminum in accordance With this invention by rst spraying thesurface with about )g6 to .O08 inch of molybdenum 47. The unit may becoated with molybdenum by cladding or electroplating instead ofspraying. An oxidation protective eeramie coating then may be appliedover the molybdenum `coating over any areas not to be covered bymolten-aluminum. Such an immersion unit has been found to provide verysatisfactory service in molten aluminum.

It is also within the scope of the invention to provide parts for pumpsand the like, used inccntastwitll, molten aluminum. made from molybdenumor tuestemmetal- The invention has been illustrated by way ainumber ofarticles or parts which commonly areused inrnolten aluminum, but it isnot mylintention to limit theinvenf tion thereto otherwise than asnecessitated `by the.ap.

pended claims. Furthermore, various modifications tof, the inventionwill become apparent to those skilled 4', th art without departing fromthe spirit of the invention,

l claim as my invention: y t

1- In combination, a refractory 4reservoir for molten aluminum and atube disposed therein below the normal level of the molten aluminum,said tube being made from a ferrous -metal coated with'an intermediatelayer Iof molybdenum and an external layer of ceramic compositionwhereby said tube is protected from corrosion `by molten aluminum andfrom oxidation.

2. In combination, a refractory reservoir `for moltenv aluminum and atube disposed therein belowthe normal level of the molten aluminum, saidtube being made from an alloy containing substantially nickel, 1,-2 to1,4% chromium and 6 to 8% iron and coated with-,anlintermediate layer ofmolybdenum and an external `layenpf` ceramic composition whereby saidtubeis protectedfram` corrosion by molten aluminum and fromoxidation.V,t

3. An aluminum melting furnace having two ichainhers` adapted to holdmolten aluminum, said chamhersibeing interconnected by a ferrous metaltube having its sui-face coated with an intermediate layer of molybdenumand an external layer of ceramic composition. f i

References Cited in the le of this patent UNITED STATES PATENTS Beidleret al Ian. 12,

OTHER REFERENCES Y Metals Handbook. 1948 edition, published by ThelAmerican Society of Metals (pages 1057-1058 relied on).

1. IN COMBINATION A REFRACTORY RESERVOIR FOR MOLTEN ALUMINUM AND A TUBEDISPOSED THEREIN BELOW THE NORMAL LEVEL OF THE MOLTEN ALUMINUM, SAIDTUBE BEING MADE FROM A FERROUS METAL COATED WITH AN INTERMEDIATE LAYEROF MOLYBDENUM AND AN EXTERNAL LAYER OF CERAMIC COMPOSITION WHEREBY SAIDTUBE IS PROTECTED FROM CORROSION BY MOLTEN ALUMINUM AND FROM OXIDATION.